JR pass covers all transportation fee for JR trains, but it does not cover subways operated by Tokyo Metro, Toei, and other train lines such as Tokyu Toyoko Line, Tokyu Denentoshi Line, etc. Suica card is a chargeable IC card allows you to get in the gate just by touching the card. It is very convenient if you are also going to use such train lines, but you can always buy tickets at the ticket machine each time it is necessary.
Deciding the period of the ticket much depends on where you are traveling. If you are traveling within Tokyo, there are many train lines operated by different companies other than JR. So JR pass may not be much useful to go around Tokyo if you want to save your time. It seems to be much useful at areas other than Tokyo, where JR is almost the only train or bus transportation company.
Buying reserved seats will not require a JR pass. We Japanese buy reserved seats without JR Pass. We buy at ticket machine of "Midori No Madoguchi" (information center at stations)